The present invention relates to correction and normalization techniques for signals from a radio frequency receiver coil of magnetic resonance scanners. This invention finds particular application in conjunction with image data correction techniques for signals from "quadrature" receiving coils which are not, in fact, in quadrature over their entire field of view and will be described with particular reference thereto.
Heretofore, various "quadrature" coils have been utilized with magnetic resonance imaging and spectroscopy equipment. The quadrature coils typically include two coils or coil arrays which view the same region of interest, but are sensitive to signals 90.degree. out of phase. Signals from the two coils are connected to an analog phase shifting circuit which causes both signals to have the same phase. Typically, the analog phase shifting circuit is an LC circuit which advances the phase of the lagging signal by 45.degree. and retards the phase of the leading signal by 45.degree. such that the two phases match. Once the phases match, the signals are summed, providing a signal to noise improvement of the .sqroot.2.
More mathematically stated, when two signals and S.sub.2 are combined, the resultant signal S.sub.a is defined by: ##EQU1## where .alpha. is the phase difference between the two signals. It is readily apparent that S.sub.a is maximized when .alpha.=90.degree. and S.sub.1 and S.sub.2 are equal, i.e., a true quadrature relationship. It will further be noticed that as the phase angle .alpha. between the signals approaches zero, the advantages of summing disappear. Moreover, as the magnitude of the signals differ, summing the two components can actually become disadvantageous.
Typically, fully circularly symmetric coils, like a birdcage coil, are in quadrature over substantially the entire region of interest. However, other coils, such as planar coils, tend to only have a plane of symmetry along which the signals received by the two coils are orthogonal. Signals from off the plane of symmetry tend to lose their orthogonality with distance from the plane of symmetry. Moreover, the intensity or relative magnitude of the signals received by the two coils from points in space differ over the field of view. When these signals are combined with a conventional analog combiner, signals originating along the line of symmetry show good intensity and signal-to-noise improvement. However, signal sources off the plane of symmetry tend to show less advantage with deviation from the plane of symmetry.
Phase angle deviations in the signals received by different coils has also proven a problem in phased array coils. In phased array coils, a plurality of coils are disposed in a line with only small regions of overlap to image an enlarged area. Image portions are combined at the regions of overlap to produce an image that is larger than the field of view of any individual coil. Phase variations at the regions of overlap tend to cause discontinuities in the image of the entire field of interest. In order to combine these images from linear coils with adjacent, slightly lapping fields of view, weighted magnitude images have been combined using a noise resistance matrix. Such image adjustment is, of course, performed after magnitude reconstruction. See, for example, U.S. Pat. No. 4,825,162.
U.S. Pat. No. 4,947,121 describes a technique for combining signals from receiver coils using noise data samples and creating a noise matrix. These noise matrix techniques require additional scan time in order to acquire data for the noise matrix. Moreover, these techniques assume that the noise values of the two coils correctly described the signal phase and magnitude deviations. When the anatomy to be imaged can affect the coil signal pattern, the signal phase and magnitude vary differently from the noise pattern with position in the field of view. Moreover, these techniques are directed to coil arrays with adjacent fields of view, not quadrature coils.
The present invention provides a new and improved quadrature signal correction technique which overcomes the above-referenced problems and others.